Hi, if you want to simulate the flow inside the cooler, dont use porosity -> from the fluent online-user-manual:

"Note that the porosity will never impact the flow velocity in the medium, [...] No matter what value you set for porosity, FLUENT predicts that the flow velocity is the superficial velocity in the medium"

you can set the fluid porosity in the porous zone properties. That is basically 1-(porosity of solid).
So for solid porosity = 0.4, you need to set fluid porosity = 0.6.
Hope this is what you are looking for.